Solid-state organic light emitting diode (OLED) displays are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the display age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials can age at different rates, causing differential color aging and a display whose white point varies as the display is used. In addition, each individual pixel can age at a different rate than other pixels resulting in display nonuniformity. Further, some circuitry elements, e.g. amorphous silicon transistors, are also known to exhibit aging effects.
The rate at which the materials age is related to the amount of current that passes through the display and, hence, the amount of light that has been emitted from the display. One technique to compensate for this aging effect in polymer light emitting diodes is described in U.S. Pat. No. 6,456,016 by Sundahl et al. This approach relies on a controlled reduction of current provided at an early stage of use followed by a second stage in which the display output is gradually decreased. This solution requires that the operating time of the display be tracked by a timer within the controller, which then provides a compensating amount of current. Moreover, once a display has been in use, the controller must remain associated with that display to avoid errors in display operating time. This technique has the disadvantage of not representing the performance of small-molecule organic light emitting diode displays well. Moreover, the time the display has been in use must be accumulated, requiring timing, calculation, and storage circuitry in the controller. Also, this technique does not accommodate differences in behavior of the display at varying levels of brightness and temperature and cannot accommodate differential aging rates of the different organic materials.
U.S. Pat. No. 6,414,661 B1 by Shen et al. describes a method and associated system to compensate for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel. The method derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, and therefore requiring complex and extensive circuitry.
U.S. Patent Application 2002/0167474 A1 by Everitt describes a pulse width modulation driver for an OLED display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction tables are calculated prior to and/or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to allow the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix. This design requires the use of a integrated, calibrated current source and A/D converter, greatly increasing the complexity of the circuit design.
U.S. Pat. No. 6,504,565 B1 by Narita et al. describes a light-emitting display which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the light-emitting element array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant. An exposure display employing the light-emitting display, and an image forming apparatus employing the exposure display are also disclosed. This design requires the use of a calculation unit responsive to each signal sent to each pixel to record usage, greatly increasing the complexity of the circuit design.
JP 2002278514 A by Numeo Koji describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured, and a temperature measurement circuit estimates the temperature of the organic EL elements. A comparison is made with the voltage value applied to the elements, the flow of current values and the estimated temperature, the changes due to aging of similarly constituted elements determined beforehand, the changes due to aging in the current-luminance characteristics, and the temperature at the time of the characteristics measurements for estimating the current-luminance characteristics of the elements. Then, the total sum of the amount of currents being supplied to the elements in the interval during which display data are displayed is changed, which can provide the luminance that is to be originally displayed, based on the estimated values of the current-luminance characteristics, the values of the current flowing in the elements, and the display data. This design presumes a predictable relative use of pixels and does not accommodate differences in actual usage of groups of pixels or of individual pixels. Hence, correction for color or spatial groups is likely to be inaccurate over time. Moreover, the integration of temperature and multiple current sensing circuits within the display is required. This integration is complex, reduces manufacturing yields, and takes up space within the display.
U.S. Patent Application 2003/0122813 A1 by Ishizuki et al. discloses a display panel driving device and driving method for providing high-quality images without irregular luminance even after long-time use. The light-emission drive current flowing is measured while each pixel successively and independently emits light. Then the luminance is corrected for each input pixel data based on the measured drive current values. According to another aspect, the drive voltage is adjusted such that one drive current value becomes equal to a predetermined reference current. In a further aspect, the current is measured while an off-set current, corresponding to a leak current of the display panel, is added to the current output from the drive voltage generator circuit, and the resultant current is supplied to each of the pixel portions. This design presumes an external current detection circuit sensitive enough to detect the current changes due to a single pixel's power usage. The measurement techniques are iterative, and therefore slow.
Arnold et al., in U.S. Pat. No. 6,995,519, teach a method of compensating for aging of an OLED device. This method assumes that the entire change in device luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from amorphous silicon (a-Si), this assumption is not valid, as the threshold voltage of the transistors also changes with use. The method of Arnold will not provide complete compensation for OLED efficiency losses in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
There is a need therefore for a more complete compensation approach for organic light emitting diode displays.